Gregarinasina

Gregarines occur in both aquatic and terrestrial environments. Although they are usually transmitted by the orofaecal route, some are transmitted with the host's gametes during copulation, e.g., Monocystis.

In all species, four or more sporozoites (depending on the species), equipped with an apical complex, escape from the oocysts in a process called excystation. They find their way to the appropriate body cavity, and penetrate host cells in their immediate environment. The sporozoites begin to feed within the host cell and develop into larger trophozoites. In some species, the sporozoites and trophozoites are capable of asexual replication – a process called schizogony or merogony.

In all species, two mature trophozoites eventually pair up in a process known as syzygy and develop into gamonts. During syzygy, gamont orientation differs between species (side-to-side, head-to-tail). A gametocyst wall forms around each gamont pair, which then begins to divide into hundreds of gametes. Zygotes are produced by the fusion of two gametes, and these, in turn, become surrounded by an oocyst wall. Within the oocyst, meiosis occurs, yielding the sporozoites. Hundreds of oocysts accumulate within each gametocyst; these are released via a host's faeces or via host death and decay.

File:Lankesteria lm.jpg is an intestinal parasite of ascidians. These aseptate gregarines lack epimerites and instead possess attachment organelles known as mucrons]]

Gregarines have thus far been reported to infect over 3000 invertebrate species.Alarcón M E., Huang C-G, Tsai Y-S, Chen W-J, Kumar A (2011) Life cycle and morphology of Steinina ctenocephali (Ross 1909) comb. nov. (Eugregarinorida: Actinocephalidae), a gregarine of Ctenocephalides felis (Siphonaptera: Pulicidae) in Taiwan. Zoological Studies 50(6): 763-772

The gregarines were recognised as a taxon by Grasse in 1953.{{cite book |first1=P.P. |last1=Grassé |first2=M.C. |last2=Caullery |title=Traité de zoologie: anatomie, systématique, biologie. Tome I, Fasc. II, Protozaires, rhizopodes, Actinopodes, Sporozoaires, Cnidosporidies |publisher=Masson et Cie |location=Paris |year=1953 |oclc=642231286 }} The three orders into which they are currently divided were created by Levine et al. in 1980.

Currently, about 250 genera and 1650 species are known in this taxon. They are divided into three orders based on habitat, host range, and trophozoite morphology.{{cite book |vauthors=Perkins FO, Barta JR, Clopton RE, Peirce MA, Upton SJ |chapter=Phylum Apicomplexa |veditors=Lee JJ, Leedale GF, Bradbury P |title=An Illustrated guide to the Protozoa: organisms traditionally referred to as protozoa, or newly discovered groups |publisher=Society of Protozoologists |edition=2nd |year=2000 |oclc=704052757 |isbn=978-1891276224 |pages=190–369 |volume=1}}

Most species have monoxenous lifecycles involving a single invertebrate host. In the lifecycle, the extracellular feeding stage is known as the trophozoite.

Archigregarines are found only in marine habitats. They possess intestinal trophozoites similar in morphology to the infective sporozoites. Phylogenetic analysis suggests this group is paraphyletic and will need division. Generally, four zoites are in each spore in this group.

Eugregarines are found in marine, freshwater, and terrestrial habitats. These species possess large trophozoites that are significantly different in morphology and behavior from the sporozoites. This taxon contains most of the known gregarine species. The intestinal eugregarines are separated into septate – suborder Septatorina – and aseptate – suborder Aseptatorina – depending on whether the trophozoite is superficially divided by a transverse septum. The aseptate species are mostly marine gregarines.

Urosporidians are aseptate eugregarines that infect the coelomic spaces of marine hosts. Unusually, they tend to lack attachment structures and form gamont pairs that pulsate freely within the coelomic fluid.

Monocystids are aseptate eugregarines that infect the reproductive vesicles of terrestrial annelids. These latter species tend to branch closely with neogregarines and may need to be reclassified. Generally, eight zoites are in each spore in this group.

Neogregarines are found only in terrestrial hosts. These species have reduced trophozoites and tend to infect tissues other than the intestine. Usually, eight zoites are in each spore in this group.

The eugregarines and neogregarines differ in a number of respects. The neogregarines are in general more pathogenic to their hosts. The eugregarines multiply by sporogony and gametogony, while the neogregarines have an additional schizogenic stage – merogony – within their hosts. Merogony may be intracellular or extracellular depending on the species.

DNA studies suggest the archigregarines are ancestral to the others.{{cite journal |author=Leander BS |title=Marine gregarines: evolutionary prelude to the apicomplexan radiation? |journal=Trends Parasitol. |volume=24 |issue=2 |pages=60–7 |date=February 2008 |pmid=18226585 |doi=10.1016/j.pt.2007.11.005 }}

Cavalier-Smith has proposed a significant revision of this taxon assuming the polyphyly of eugregarines.{{cite journal |vauthors=Cavalier-Smith T |title=Gregarine site-heterogeneous 18S rDNA trees, revision of gregarine higher classification, and the evolutionary diversification of Sporozoa |journal=Eur. J. Protistol. |volume=50 |issue=5 |pages=472–495 |year=2014 |pmid=25238406 |doi=10.1016/j.ejop.2014.07.002 }} He has separated gregarines into three classes. The first of them – Gregarinomorphea – comprises Orthogregarinia, Cryptosporidiidae and, additionally, Rhytidocystidae previously considered as divergent coccidians or Apicomplexa incertae sedis.{{cite journal |vauthors=Adl SM, Simpson AG, Lane CE, Lukeš J, Bass D, Bowser SS, Brown MW, Burki F, Dunthorn M, Hampl V, Heiss A, Hoppenrath M, Lara E, Le Gall L, Lynn DH, McManus H, Mitchell EA, Mozley-Stanridge SE, Parfrey LW, Pawlowski J, Rueckert S, Shadwick L, Shadwick L, Schoch CL, Smirnov A, Spiegel FW |title=The revised classification of eukaryotes |journal=J. Eukaryot. Microbiol. |volume=59 |issue=5 |pages=429–93 |year=2012 |pmid=23020233 |pmc=3483872 |doi=10.1111/j.1550-7408.2012.00644.x }} The Orthogregarinia with two new orders Arthrogregarida and Vermigregarida was created for the gregarines most closely related to Cryptosporidium. The second class – Paragregarea – was created for the archigregarines, Stenophorida and a new order – Velocida which itself was created for Urosporoidea superfam. n. and Veloxidium. The third class was created – Squirmida – for Filipodium and Platyproteum. Thus, the eugregarines proved to be split and distributed among these three classes together with some other apicomplexans.

This point of view was challenged in 2017 by Simdyanov and co-authors, who performed the global integrated analysis of available morphological and molecular phylogenetic data and concluded that eugregarines are rather a monophyletic taxon.{{cite journal |vauthors=Simdyanov TG, Guillou L, Diakin AY, Mikhailov KV, Schrével J, Aleoshin VV |title=A new view on the morphology and phylogeny of eugregarines suggested by the evidence from the gregarine Ancora sagittata (Leuckart, 1860) Labbé, 1899 (Apicomplexa: Eugregarinida) |journal=PeerJ |volume=5 |pages=e3354 |year=2017 |pmid=28584702 |pmc=5452951 |doi=10.7717/peerj.3354 |doi-access=free }}

Several genera of gregarines are currently not classified: Acuta, Cephalolobus, Gregarina, Levinea, Menospora, Nematocystis, Nematopsis, Steinina, and Trichorhynchus.

• Meiosis occurs in all species.
• Monoxenous – only one host occurs in lifecycle for almost all species.
• Mitochondria have tubular cristae and are often distributed near the cell periphery.
• Apical complex occurs in the sporozoite stage, but is lost in the trophozoite stage in eugregarines and neogregarines.
• Trophozoites have a large and conspicuous nucleus and nucleolus.
• They inhabit extracellular body cavities of invertebrates such as the intestines, coeloms, and reproductive vesicles.
• Attachment to host occurs by a mucron (aseptate gregarines) or an epimerite (septate gregarines); some gregarines (urosporidians) float freely within extracellular body cavities (coelom).

The parasites are relatively large, spindle-shaped cells, compared to other apicomplexans and eukaryotes in general (some species are {{nowrap|> 850 µm}} in length). Most gregarines have longitudinal epicytic folds (bundles of microtubules beneath the cell surface with nematode like bending behaviour): crenulations are instead found in the urosporidians.

The gregarines are able to move and change direction along a surface through gliding motility without the use of cilia, flagella, or lamellipodia.{{cite journal |vauthors=Walker MH, Mackenzie C, Bainbridge SP, Orme C |title=A study of the structure and gliding movement of Gregarina garnhami |journal=J Protozool |volume=26 |issue= 4|pages=566–574 |date=November 1979 |doi=10.1111/j.1550-7408.1979.tb04197.x }} This is accomplished through the use of an actin and myosin complex.{{cite journal |author=Heintzelman MB |title=Actin and myosin in Gregarina polymorpha |journal=Cell Motil. Cytoskeleton |volume=58 |issue=2 |pages=83–95 |date=June 2004 |pmid=15083530 |doi=10.1002/cm.10178 }} The complexes require an actin cytoskeleton to perform their gliding motions.{{cite journal |vauthors=Mitchison TJ, Cramer LP |title=Actin-based cell motility and cell locomotion |journal=Cell |volume=84 |issue=3 |pages=371–9 |date=February 1996 |pmid=8608590 |doi=10.1016/s0092-8674(00)81281-7 |doi-access=free }} In the proposed ‘capping’ model, an uncharacterized protein complex moves rearward, moving the parasites forward.{{cite journal |vauthors=Sibley LD, Hâkansson S, Carruthers VB |title=Gliding motility: an efficient mechanism for cell penetration |journal=Curr. Biol. |volume=8 |issue=1 |pages=R12–4 |date=January 1998 |pmid=9427622 |doi=10.1016/S0960-9822(98)70008-9 |doi-access=free }}

{{Clear}}

The gregarines are among the oldest known parasites, having been described by the physician Francesco Redi in 1684.{{cite book |last=Redi |first=F |title=Osservazioni intorno agli animali viventi, che si trovano negli animali viventi |publisher=P. Matini |location=Firenze |year=1684 |oclc=8705660 }}

The first formal description was made by Dufour in 1828. He created the genus Gregarina and described Gregarina ovata from Folficula aricularia. He considered them to be parasitic worms. Koelliker recognised them as protozoa in 1848.

{{Reflist|2}}

• {{cite book |first1=Isabelle |last1=Desportes |first2=Joseph |last2=Schrével |title=Treatise on Zoology — Anatomy, Taxonomy, Biology. The Gregarines (2 vols): The Early Branching Apicomplexa |url=https://books.google.com/books?id=YxNOPQN7LJQC |date=2013 |publisher=Brill |isbn=978-90-04-25605-7}}

{{EB1911 Poster|Gregarines}}

• [http://tolweb.org/Gregarina/124806 Tree of Life Gregarina]
• [http://tolweb.org/movies/Gregarina/124806 Gregarina Movies]

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Category:Apicomplexa subclasses

Category:Conoidasida